1. Field of the Invention
The present invention relates to a device for detecting knockings generated in an internal combustion engine to produce an electrical output which can be fed, for example, into an engine spark timing control system to optimize the engine spark timing. More particularly, the invention is concerned with an engine knocking detection device of the type which is provided with a diaphragm exposed to the engine cooling water so that the diaphragm resonates with high frequency pulsations of the water pressure which are caused by vibrations of engine cylinders which in turn are generated by engine knockings which occasionally take place in engine cylinders.
2. Description of the Prior Art
With the knocking detection device of the type specified above, air bubbles tend to be adhered or attached to the surface of the diaphragm exposed to the engine cooling water. Since air bubbles are compressible, the pulsations of the engine cooling water pressure caused by engine knockings are inconveniently absorbed by the air bubbles, so that the amplitude of the vibration of the diaphragm is reduced to undesirably lower the output voltage of the knocking detection device. This tendency is remarkably increased especially when the engine knocks at a high frequency range. It is also experienced that, with certain diameters of air bubbles, a diaphragm which provides a generally flat frequency characteristic before the attachment of the air bubbles thereto exhibits a resonance after the attachment of the air bubbles.